Pyrolysis processes

ABSTRACT

THERE IS DISCLOSED A METHOD TO IMPROVE THE SELECTIVITY OF A PROCESS WHEREBY CERTAIN OLEFINS WHICH ARE DIOLEFIN PRECURSORS, ARE DEMETHANATED OR DE-ETHANATED BY PYROLYSIS TO PRODUCE CERTAIN DIOLEFINS. THE IMPROVEMENT CONSISTS OF ADDING TO THE DIOLEFIN PRECURSORS SMALL AMOUNTS OF DIOLEFINS, CONTAINING THE SAME NUMBER OF CARBON ATOMS AND SIDE CHAINS IN THE SAME POSITION, IF ANY, AS THE DIOLEFIN PRECURSORS, PRIOR TO THE PYROLYSIS.

United States Patent C 3,637,886 PYROLYSIS PROCESSES Kenneth J. Frech, 480 Greenfield Circle, Tallmadge, Ohio 44278 No Drawing. Filed Sept. 22, 1967, Ser. No. 669,723 Int. Cl. C07c 3/58 US. Cl. 260-680 C 7 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a method to improve the selectivity of a process whereby certain olefins which are diolefin precursors, are demethanated or de-ethanated by pyrolysis to produce certain diolefins. The improvement consists of adding to the diolefin precursors small amounts of diolefins, containing the same number of carbon atoms and side chains in the same position, if any, as the diolefin precursors, prior to the pyrolysis.

This invention relates to improvements in pyrolysis processes. It relates to a process by which the efficiency and yield to the diolefin is improved when a diolefin precursor is demethanated by pyrolysis to form a diolefin. The improvement comprises adding to the diolefin precursor feedstock certain diolefin or diolefins containing the same number of carbon atoms as does the diolefin precursor.

It is known that certain olefins, referred to as diolefin precursors, may be thermally decomposed by subjecting them to pyrolysis at elevated temperatures. For instance, diolefin precursors which have in their molecular make-up a carbon-to-carbon single bond which is in a position beta to the double bond, under the proper conditions of temperature, time and pressure, undergo a scission of the carbon to carbon single bond which is in a position beta to the double bond. This scission results in the olefin molecule being split into two or more fragments. When the smallest of these fragments contains only one carbon atom, this process is usually referred to as demethanation and when the smallest fragment contains two carbon atoms, it is called de-ethanation. The two fragments will stabilize themselves to form the desired diolefin and either methane or ethane.

For instance, it is known that Z-pentene, an olefin containing five carbon atoms can be demethanated by pyrolysis to form the diolefin, 1,3-butadiene. Likewise, hexenes such as 2-ethyl-1-butene, 2-methyl-2-pentene, and 3-methyl-2-pentene can be demethanated by pyrolysis to form 2-methyl-l,3-butadiene or isoprene. Demethanation by pyrolysis of 4-methyl-2-pentene and 3-hexene results in the formation of piperylene. Also, 3-ethyl-2-pentene and 2-ethyl-2-pentene can be demethanated by pyrolysis to form 2-ethyl-1,3-butadiene. The precursor, 2-hexene, can be de-ethanated to form 1,3-butadiene.

It has been observed that when these diolefin precursors are subjected to pyrolysis conditions, in order to be demethanated or de-ethanated to form the desired diolefins, small amounts of the precursors undergo, what appears to be, dehydrogenation rather than demethanation or deethanation. It is believed that this dehydrogenation occurs because of a surface catalysis on the walls of the pyrolysis reactor. Therefore, as a result of this small amount of dehydrogenation, the efliciency of the pyrolysis process to the desired diolefin is reduced somewhat. This is because these small amounts of the precursor are utilized to produce another undesired diolefin instead of the desired diolefin. This small loss in the proper use of the starting precursor results in a slightly less efiicient process.

It has now been discovered that improvements in the efiiciency and yields to the desired diolefins can be ob- 3,637,886 Patented Jan. 25, 1972 tained in processes wherein diolefins are produced by the demethanation and de-ethanation of diolefin precursors. This improvement comprises adding to the diolefin precursors small amounts of certain diolefins containing the same number of carbon atoms and side chains, if any, in the same position as do the precursors, prior to the precursors being subjected to pyrolysis in order to demethanate or de-ethanate them.

For instance, there can be obtained an improvement in the efficiency to 2-methyl-1,3-butadiene or isoprene when 2-methyl 2-pentene is demethanated if small amounts of 4-methyl-l,3-pentadiene and 2-methyl-1,3-pentadiene are added to the 2-methyl-2-pentene prior to pyrolysis. Likewise, the efliciency to isoprene by the demethanation of 3-methyl-2-pentene can be enhanced if small amounts of cis and trans 3-methyl-1,3-pentadiene is added to the 3- methyl-Z-pentene prior to demethanation. The efficiency of the demethanation of Z-ethyl-l-butene to isoprene is enhanced also if small amounts of cis and trans 3-methyl- 1,3-pentadiene are added prior to demethanation of 2- ethyl-l-butene. y

In the formation of butadiene by the demethanation of 2-pentene improvements in efiiciency can be obtained if cis and trans piperylene are added to the Z-pentene prior to demethanation. In the formation of butadiene by the de-ethanation of 2-hexene, the efficiency can be improved if small amounts of 1,3-hexadiene and cis and trans 2,4- hexadiene are added prior to de-ethanation of 2-hexene.

Increases in efliciency to piperylene by the demethanation of 4-methyl-2-pentene can be obtained if small amounts of 4-methyl-[1,3-pentadiene and 2-methyl-1,3- pentadiene are added prior to the pyrolysis of 4-methyl- Z-pentene. Also, improvements can be Obtained in efficiency to piperylene from 3-hexene if 1,3-hexadiene and cis and trans 2,4-hexadiene in small amounts, are added prior to the pyrolysis of 3-hexene.

Improvements in the efiiciency of the demethanation of 2-ethyl-2-pe11tene to form 2-ethyl-l,3-butadiene are obtained if small amounts of 2-ethyl-1,3-pentadiene are added prior to the demethanation step. Likewise, improvements in efficiencies to 2-ethyll,3-butadiene from 3-ethyl- Z-pentene are obtained if small amounts of 3-ethyl-1,3- pentadiene are added prior to the demethanation.

In the decomposition of the diolefin precursors previously mentioned, conventional procedures are employed. It is generally preferred to demethanate or de-ethanate these precursors at temperatures ranging between about 500 C. and 900 C. and more generally preferred to employ temperatures ranging from about 625 C. and 775 C.

The time that these diolefin precursors are in the cracking zone may range broadly from about 0.001 to about 2 seconds, however, more preferable times are from about 0.05 to about 0.5 second. These times are referred to as residence times and are usually defined as the time required for one molecule of the diolefin precursor to pass through the pyrolysis zone.

Generally, the olefins are fed to the pyrolysis reactor in mixture with some inert diluent, however, they can be pyrolyzed without the use of diluents. Suitable diluents are inert materials such as steam, and other hydrocarbons which are inert to the demethanation or de-ethanation of the diolefin precursors, such as parafiins, such as methane, ethane and the like. Steam is usually preferred because of economy and ease of separation of the efiluent. The ratio of the diluent to the olefin may vary widely. From about 0.5/1 to about 15/1 moles of diluent per mole of diolefin precursor may be used, however, a more realistic ratio ranges from about 2/1 to about 4/1.

Pressures employed are not critical and may vary from a low pressure from about 10 milliliters of mercury to about 500 pounds per square inch with somewhere between atmospheric to about 100 pounds per square inch being more preferred.

Also, it is contemplated within the scope of this invention that certain cracking promoters also be employed in the pyrolysis to aid in the demethanation step. Representative of such suitable promoters are hydrogen sulfide, hydrogen bromide, ammonium bromide, allyl bromide, methyl mercaptan, methylene chloride, refractory olefins, ammonium sulfide, ammonium hydrosulfide, the reaction product of one mole of hydrogen sulfide and one or two moles of an amine and mixtures of these cracking promoters with aromatic hydrocarbons such as benzene and toluene. Mixtures of these cracking promoters is also contemplated in this invention.

The amount of diolefin containing the same number of carbon atoms as the diolefin precursor to be added to the diolefin precursor prior to the pyrolysis step can be varied. Obviously, sufficient amounts should be added to cause the desired effect. This amount can be considered the smallest amount to add. While there is no theoretical upper limit to the amount that can be added, there is a practical limitation which probably should be observed, since to add a large amount of the diolefin prior to the pyrolysis would be uneconomical in view of the benefits gained. As a practical matter, it has been observed that about 2 weight percent of the diolefin containing the same number of carbon atoms as the precursor should be added to gain sufiicient benefit to make the whole effort worthwhile. The practical upper limit seems to be about weight percent. As can be seen from the specific embodiment that the greatest advantage in the experiments performed was obtained when around 6 by weight was employed. The optimum, of course, would depend on many factors which must be taken into consideration by those skilled in the art of the pyrolysis of diolefin precursors to form diolefins. Weight percent is used.

The invention is further illustrated by reference to the trative and not limiting of this invention. Weight percent is used.

EXAMPLE I In the following example, a continuous pyrolysis of a feedstock composed predominantly of 2-methyl-2-pentene with about 3% each of 2-methyl-1-pentene and 4-methyl- 2-pentene, the invention is illustrated by varying the amounts of a mixture, 50% by weight, of 4-methyl-l,3- pentadiene and Z-methyl 1,3 pentadiene added to the feedstock prior to pyrolysis. A diluent was employed which was steam in a mole ratio of steam/2-methyl-2- pentene of 2.5/1. The average pressure in the pyrolysis reactor was pounds per square inch gauge. A cracking promoter, ammonium sulfide, was employed at a level of 10 mole percent based on the moles of 6 carbon olefins in the feedstock. While there was an attempt to maintain the temperature and residence time constant, they did vary slightly as indicated in the table. As a result of this slight variation, the conversion varied slightly from experiment to experiment and the actual results which are given in column E were normalized to 50% conversion as reported in column F and the net methyl pentadienes in the product reported in column G are based on the results which were normalized to 50% conversion.

In the following table column A is the experiment number; column B is the temperature in degrees C.; column C is residence time in seconds; column D is methyl pentadienes in feedstock in Weight percent; column E is actual gross methyl pentadiene in product in weight percent; column F is gross methyl pentadienes in product, normalized to 50 percent conversion in weight percent; and column G is net methyl pentadiene in the product normalized to 50 percent conversion in weight percent. The net methyl pentadiene values are obtained by subtracting the amount of methyl pentadienes added 4 to the precursor from the gross methyl pentadienes found in the product stream normalized to 50% conversion.

Please note that under approximately the same conditions, the gross amount of the dehydrogenation products, in this example, methyl pentadiene, is lowered from 8.8 weight percent to 2.3 weight percent when only 6.6 weight percent of methyl pentadiene is added to the feedstock.

The experiments above illustrate that in a process whereby olefins which are diolefin precursors are demethanated by pyrolysis to form diolefins, there is obtained a reduction in the amount of the dehydrogenation product produced, when small amounts of a diolefin, containing the same number of carbon atoms and side chains, if any, as does the diolefin precursor, is added to the diolefin precursor prior to the pyrolysis. Thus, this process seems to provide an opportunity to not only lower the net make of the undesirable dehydrogenation product, but also allows more of the diolefin precursor to be available to be converted into the desired diolefin by demethanation or de-ethanation. This process also provides a method to modify the composition of the effiuent in such pyrolysis processes.

EXAMPLE II In this example, a series of three continuous pyrolysis experiments were conducted using a feedstock which was composed of hexenes. This feedstock was a mixture of about 88.3 weight percent of a mixture comprising 3 weight percent of 2-methyl-1-pentene and 97 weight percent of 2-rnethyl-2-pentene. The remainder of the feedstock comprised a mixture of about 50 percent by weight of 4-methyl-2-pentene and about 50 percent by weight of a mixture of 2 and 3-hexenes. The steam was employed as a diluent at a mole ratio of H O/hydrocarbon of 3/ 1. Ammonium sulfide (NH S in the amount of 10 mole percent based on the total hexenes in the feedstock was employed as a pyrolysis promoter. The conditions of pyrolysis were a residence time of 0.18 second and the temperature averaged 660 C. for all three runs. The first run was considered to be a control having no added methyl pentadienes. The second and third runs had added to them varying amounts of a 50/50 by weight mixture of 2-rnethyl-1,3 pentadiene and 4-methyl-l,3-pentadiene. The results are given in the table below in which column 1 is the run number; column 2 is the amount of methyl pentadienes added; column 3 is the yield per pass in mole percent of isoprene; column 4 is the mole percent conversion obtained and column 5 is the reaction selectivity or efficiency in percent.

ascertained that improvements in both mole percent yield per pass and reaction selectivity or reaction efiiciency is obtained when diolefins containing the same number of. carbon atoms and side chains, if any, as do the precursors, are added to the diolefin precursors prior to pyrolysis.

Note that when 5.0% by weight of methylpentadienes are added to the feedstock, it results in an almost 11% increase in the reaction selectivity.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

1. In the process of producing isoprene which comprises pyrolyzing Z-methyI-Z-pentene under pyrolysis conditions, the improvement comprising adding to said 2-methyl-2- pentene, prior to pyrolysis, about 2 to about 10 weight percent of at least one diolefin selected from the group consisting of 4-methyl1,3-pentadiene and 2-methyl-l,3- pentadiene.

2. In the process of producing isoprene which comprises pyrolyzing 3-methyl-2-pentene under pyrolysis conditions, the improvement comprising adding to said 3-methyl-2- pentene, prior to pyrolysis, about 2 to about 10 Weight percent of at least one diolefin selected from the group consisting of cis 3-methyl-l,3-pentadiene and trans 3- methyl-1,3-pentadiene.

3. In the process of producing isoprene which comprises pyrolyzing 2-ethyl-1-butene under pyrolysis conditions, the improvement comprising adding to said 2-ethyll-butene, prior to pyrolysis, about 2 to about 10 weight percent of at least one diolefin selected from the group consisting of cis-3-methyl-l-pentadiene and trans 3-methyl-l,3-pentadiene.

4. In the process of producing 2-ethy1-1,3-butadieue which comprises pyrolyzing 2 ethyl 2-pentene under pyrolysis conditions, the improvement comprising adding to said 2-ethyl-2-pentene, prior to pyrolysis, about 2 to about 10 Weight percent of the diolefin 2-ethyl-1,3-pentadiene.

5. In the process of producing 2-ethyl-l,3-butadiene which comprises pyrolyzing 3-ethyl-2-pentene under pyrolysis conditions, the improvement comprising adding to the said 3-ethyl-2-pentene, prior to pyrolysis about 2 to about 10 Weight percent of the diolefin 3-ethyl-1,3pentadiene.

6. In the process of producing piperylene which comprises pyrolyzing 4 methyl-2-pentene, under pyrolysis conditions, the improvement comprising adding to said 4- methyl-2-pentene, prior to pyrolysis, from about 2 to about 10 percent of the diolefin selected from the group of 4-methyl-1,3-pentadiene and 2-methyl1,3-pentadiene.

7. In the process of producing piperylene which comprises pyrolyzing 3-hexene, under pyrolysis conditions, the improvement comprising adding to said 3-hexene, prior to pyrolysis, from about 2 to about 10 percent of a diolefin from the group consisting of 1,3-hexadiene and cis and trans 2,4-hexadiene.

References Cited UNITED STATES PATENTS 2,391,158 12/1945 Hepp 260-68O C FOREIGN PATENTS 832,475 4/1960 Great Britain 260-680 C PAUL M. COUGHLAN, JR., Primary Examiner TEE STATES PATENT oTTTtE Po-mbo T (5/69) QERTIFECATE OF (IERRECTEfiN Patent No 3a 37a8 Dated January 59 97 Inventor(s) Kenneth J Frech It is certified that error appears in the above-identified'patent and that said Letters Patent are hereby corrected as shown below:

Column 1, the heading on the patent which now reads:

3 37 9 PTEQETSIS PROCESSES Kenneth J Frech, +80 Greenfield Circle Tallmadge, Ohio +278 No Drawing. Filed Sept 22, 1967, SerQ No. 669,723

, Int 01., C070 3/58 U, S0 C1. 260====68OC 7 Claims Should read:

3 3% PYEOLYSIS PROCESSES Kenneth J Frech, +8O Greenfield Circle Tallmadge, Ohio M278, Assignor to The Goodyear Tire & Rubber Co Akron, Ohio No Drawingo Filed Sept 22, 197, Ser. No, 669,723

Int C1. 0070' 3/58 7 U0 Sm C10 260--68OC 7 Claims Column 2., line 31, +.-methyl-[l,3-pentadiene Should read +-methyl-l,3 pentadiene.

Signed and sealed this lth day of August 1972.

(SEAL) Attest:

ROBERT GOTTSGHALK Commissioner of Patents EDWARD MoFLEICHER,JILv Attesting Officer 

